1. Field of the Invention
The invention relates to the field of transformers having closed ferrite magnetic circuits with an air gap, and in particular, to arrangements for reducing magnetostrictive vibrations by filling the air gap.
2. Description of Related Art
Transformers having closed ferrite magnetic circuits with an air gap are used notably in switching power supplies. The reason is that such power supplies operate in accumulation mode or "fly-back" mode requiring storage of energy in the ferrite magnetic circuit of a transformer before transferring this energy to the electrical circuit fed by the transformer. This energy storage necessitates the use of a magnetic circuit having an air gap. Traditionally, a transformer used in a switching power supply comprises a magnetic circuit consisting of two ferrite half-circuits whose section is "E-shaped", and a frame consisting of one or more electrically insulated coils. This frame has an axial hole which receives the central "leg" of the "E". The magnetic circuit and the frame are assembled by gluing. Preferably, one of the ferrite half circuits is glued to the frame, then the two half-circuits are assembled by gluing together the ends of their outer legs. This arrangement leaves an air gap between the inner legs and guarantees good mechanical strength for the magnetic circuit and the coils, since the air gap easily absorbs mechanical stresses, notably those due to thermal shocks according to current standards. However, in the presence of an air gap, magnetostrictive phenomena cause audible buzzing of the transformer. This magnetostrictive is a deformation effect related to the magnetization cycle. The geometry of the ferrite varies with the magnetic induction.
Suggestions for correcting this problem include filling the air gap with various types of flexible material in order to absorb the magnetostrictive vibrations caused by the opposing faces of the ferrite material defining the air gap in the magnetic circuit.
German application DE AS 1 245 487 suggests that air gaps or layers of non magnetic material be filled with a strongly adhesive hardenable material, particularly a thixotropic hardenable epoxy or polyurethane resin. Alternatively, a mechanical damping layer formed from a glass fabric impregnated with a flexible lacquer is suggested.
German application DE OS 36 11 906 suggests filling an air gap with an epoxy resin of a particular composition. A ceramic platelet is embedded in the resin. The entire filler mass is said to have a coefficient of thermal expansion adapted to that of ferrite.
French patent application FR. 87 15207, in the name of the company Orega Electronique et Mecanique, and corresponding to EP 0 390 643 and U.S. Pat. No. 5,025,241, proposes filling the gap with a silicone-type material. In order to assure the shock absorbing effect in a transformer working at variable frequencies, a one component mastic material filling the gap should have a Shore hardness that is stable with temperature, for example a polysulfphide or polyurethane mastic.
Although these materials produce good results when the gap is narrow, defined herein as being less than approximately 1 mm, such materials are less effective for larger gaps, because they must be soft enough to absorb vibrations, yet hard enough to limit the amplitude of these vibrations. None of the materials noted provides adequate characteristics in the case of large gaps.